The present invention relates generally to the field of inkjet printing and, more particularly, to the delivery of ink to inkjet print heads.
Inkjet technology is relatively well developed. The basics of this technology are described by W. J. Lloyd and H. T. Taub in "Ink-jet Devices," Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988) and in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994).
The typical thermal inkjet print head has an array of precisely formed nozzles attached to a print head substrate that incorporates an array of firing chambers that receive liquid ink (i.e., colorants dissolved or dispersed in a solvent) from an ink reservoir. Each chamber has a thin-film resistor, known as a "firing resistor", located opposite the nozzle so ink can collect between it and the nozzle. When electric printing pulses heat the thermal inkjet firing resistor, a small portion of the ink near it vaporizes and ejects a drop of ink from the print head. The nozzles are arranged in a matrix array. Properly sequencing the operation of each nozzle causes characters or images to form on the paper as the print head moves past the paper.
Air that is trapped in print cartridges has become an increasingly troublesome problem. In the past the accumulation of air in print cartridges was mainly ignored because the cartridges were large and could easily warehouse the air and because the cartridges had short operating lives and significant amounts of air did not accumulate. However, in today's advanced print cartridge designs the passage ways, particle filters, orifices, and conduits have become smaller and smaller. With these smaller dimensions air and air bubbles tend to block the flow of ink through the print cartridge and cause the nozzles not to eject ink. This leads to failure of the print cartridge and to require its premature replacement.
Air becomes entrapped in print cartridges from a plurality of sources. Initially air is present because it was not fully purged during manufacturing. Secondly, air bubbles may have been present during assembly in the ink tubes connecting the print head with the ink reservoir. After manufacture and for the life of the print cartridge, any dissolved air in the ink comes out of solution as bubbles. Further, air permeates into the print cartridge through the ink containment materials. Finally, in some circumstances air may be ingested into the print cartridge through the nozzles.
For a myriad of reasons the presence of air and air bubbles in inkjet print cartridges, which was previously ignored now dictates that air management become one of the factors influencing modern inkjet cartridge design.
One system for removing air from an inkjet print cartridge is described in U.S. Pat. No. 4,968,998 to Allen issued on Nov. 6, 1990.